1. Field of the Invention
The invention relates to shaping a bell end on a pipe of thermoplastic material by shaping the preheated pipe end on a shaping mandril, and upsetting the pipe end so that a bell end is formed having an annular sealing chamber. The inner diameter of the bell end increases in a series of steps from the diameter of the pipe to a maximum value, then decreases to the end of the bell, the wall thickness of the bell end being at least equal to the wall thickness of the pipe. Part of the inner wall of the sealing chamber serves as a sealing face.
2. Description of the Prior Art
An apparatus for forming this kind of bell end is already known from U.S. Pat. No. 3,893,794. Using this known apparatus, the upsetting operation compensates for the reduction of the wall thickness of the bell end caused by the radial enlargement, so that the provision of a reinforcement sleeve on the end of the pipe or the thickening in advance of the pipe end is a separate operation can be dispensed with. With this known apparatus, the wall thickness will be substantially the same over the entire length of the bell end; because the wall stress caused by the pressure of material flowing in the pipe is a function of the inner diameter of the bell end, an irregular stress distribution will occur in the bell end wall if the wall thickness is not varied along with the diameter. Because a constant bell end wall thickness would be calculated for the highest stress occurring, given bell end portions, where this maximum stress does not occur, will be overproportioned.